The long-term objective of these studies is to understand how retroviruses that do not possess oncogenes cause neoplastic diseases in a tissue-specific manner. An important requirement for murine leukemia virus (MLV) disease specificity is efficient replication in the target cell for transformation. The control of cell type-specific virus replication and its effect on the early stages of tumorigenesis are not well understood for MLVs that induce thymic lymphoma, such as the mink cell focus-forming (MCF) MLV. A major determinant of MCF lymphomagenicity and virus replication in thymocytes is the region in the long terminal repeat that is located downstream of the enhancer (DEN). The binding site in DEN for the transcription factor NF-kB is essential for the effect of DEN on viral pathogenicity. Aim 1 includes experiments to identify the thymic cell type that is infected by wild type and mutant MCF13 MLVs that are missing either the complete DEN region of the LTR or the NF-kB site in DEN. In Aim 2, inducible transcription factors that activate DEN will be identified to test the hypothesis that MLV infection of thymocytes results in cellular activation. Aim 3 includes experiments to examine the effects of virus infection on the proliferation of the thymic target for transformation. Proliferation will be assessed by measuring the fraction of virus-infected cells in the S and G2/M phases of the cell cycle. Other indicators of thymic cell proliferation, such as cdk2 kinase activity, and the expression of genes such as IL-2 and IL-2R, and those involved in thymocyte apoptosis will be assayed. In Aim 4, the viral genes that are responsible for thymocyte activation and/or proliferation will be identified by the construction of chimeric viruses. The results of these studies will contribute to an understanding of the control of retrovirus replication in the early stages of lymphomagenesis. They will further help elucidate the steps in T lymphocyte activation in nomal and malignant cells.